FIG. 12 is an overall block diagram of a load drive device including a conventional power supply control device. Referring to FIG. 12, a load drive device 200 includes a DC power supply 210, system main relays 220 to 240, a limiting resistor 250 connected in series to system main relay 240, an inverter 260 receiving supply of electric power from DC power supply 210 and driving a motor-generator 270, a capacitor 280 smoothing an input voltage of the inverter, and a control device 290.
System main relay 240 includes a power transistor and a diode connected thereto in an anti-parallel manner. System main relay 240 and limiting resistor 250 connected in series are connected in parallel to system main relay 230.
Here, DC power supply 210, system main relays 220 to 240, limiting resistor 250, and control device 290 form the power supply control device in load drive device 200, and inverter 260, motor-generator 270 and capacitor 280 form the load device receiving supply of electric power from the power supply control device in load drive device 200.
When a start instruction of load drive device 200 is given to control device 290, control device 290 initially turns on system main relay 220 and the power transistor of system main relay 240. Then, a circuit from a positive electrode to a negative electrode of DC power supply 210 through system main relay 220, capacitor 280, system main relay 240, and limiting resistor 250 is formed, and charging from DC power supply 210 to capacitor 280 is started.
Here, as limiting resistor 250 is provided in load drive device 200, an inrush current from DC power supply 210 to capacitor 280 is prevented, and welding of system main relays 220, 230 can be prevented.
As charging of capacitor 280 proceeds, control device 290 turns on system main relay 230, and thereafter turns off the power transistor of system main relay 240.
Thus, in the power supply control device above, generation of the inrush current is prevented by providing limiting resistor 250, however, limiting resistor 250 that is used only at the time of start of the system has been one of factors to increase the cost.
Japanese Patent Laying-Open No. 5-111240 proposes a power supply control device capable of preventing an inrush current without providing a limiting resistor. Japanese Patent Laying-Open No. 5-111240 discloses a technique to prevent the inrush current while suppressing an effective value of a flow-in current into a DC-DC converter by causing a transistor provided between a main battery and the DC-DC converter to perform PWM (Pulse Width Modulation) operation.
Meanwhile, Japanese Patent Laying-Open No. 2003-92807 discloses a technique to charge a capacitor with a constant current using a precharge circuit provided between a battery for driving and an inverter and to protect a transistor within the precharge circuit by reducing the constant current when a temperature of the transistor within the precharge circuit attains a temperature equal to or higher than a certain level.
The technique disclosed in Japanese Patent Laying-Open No. 5-111240, however, remains as the technique simply to cause the transistor to perform PWM operation in order to prevent the inrush current, and maximum rating, a temperature of the transistor and the like are not considered.
In addition, the precharge circuit disclosed in Japanese Patent Laying-Open No. 2003-92807 has a complicated circuit configuration, and reduction in cost is not sufficient.